# std::filesystem::resize_file

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< cpp‎ | filesystem

Defined in header `<filesystem>` |  |
void resize_file( const std::filesystem::path& p, std::uintmax_t new_size ); | (1) | (since C++17)
void resize_file( const std::filesystem::path& p, std::uintmax_t new_size, std::error_code& ec ) noexcept; | (2) | (since C++17)

Changes the size of the regular file named by p as if by POSIX `truncate`: if the file size was previously larger than new_size, the remainder of the file is discarded. If the file was previously smaller than new_size, the file size is increased and the new area appears as if zero-filled.

### Parameters

p | - | path to resize
new_size | - | size that the file will now have
ec | - | out-parameter for error reporting in the non-throwing overload

### Return value

(none)

### Exceptions

Any overload not marked `noexcept` may throw std::bad_alloc if memory allocation fails.

1) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed with p as the first path argument and the OS error code as the error code argument.
2) Sets a std::error_code& parameter to the OS API error code if an OS API call fails, and executes ec.`clear`() if no errors occur.

### Notes

On systems that support sparse files, increasing the file size does not increase the space it occupies on the file system: space allocation takes place only when non-zero bytes are written to the file.

### Example

Demonstrates the effect creating a sparse file has on the free space.

Run this code

    #include <filesystem>
    #include <fstream>
    #include <iostream>
    #include <locale>

    int main()
    {
        auto p = std::filesystem::temp_directory_path() / "example.bin";
        std::ofstream{p}.put('a');
        std::cout.imbue(std::locale{"en_US.UTF8"});
        std::cout << "File size:  " << std::filesystem::file_size(p) << '\n'
                  << "Free space: " << std::filesystem::space(p).free << '\n';
        std::filesystem::resize_file(p, 64*1024); // resize to 64 KB
        std::cout << "File size:  " << std::filesystem::file_size(p) << '\n'
                  << "Free space: " << std::filesystem::space(p).free << '\n';
        std::filesystem::remove(p);
    }

Possible output:

    File size:  1
    Free space: 42,954,108,928
    File size:  65,536
    Free space: 42,954,108,928

### See also

file_size (C++17) | returns the size of a file (function)
space (C++17) | determines available free space on the file system (function)

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